Integrated circuits and discrete semiconductor devices are typically fabricated on wafers made from semiconductor substrates such as, for example, silicon, silicon germanium, gallium arsenide, indium phosphide, and the like. The semiconductor wafers have either a positioning notch or a flat edge for aligning the wafers and scribe marks for individual wafer identification. Scribe marks are small alphanumeric sequences formed on the wafer by chemical etching, laser cutting, or other techniques capable of making a well defined mark on a semiconductor wafer. These marks are located close to the positioning notch or the flat edge of the semiconductor wafer. Because scribe marks are used for identifying individual semiconductor wafers, it is very important for semiconductor device manufacturers to be able to read the scribe marks.
One technique for reading scribe marks is described in U.S. Pat. No. 5,933,521 and involves moving a cassette passed a fixed reading position of a single stationary camera. Another technique for reading scribe marks is described in U.S. Pat. No. 5,553,168 and involves using a camera to capture a scribe mark image which is then converted from a video signal to a digital signal by a converter. A computer is then used to interpret the digital signal. These scribe readers have a large number of moving parts, use very sophisticated circuitry, and are expensive to maintain and operate.
Accordingly, what is needed is an apparatus for reading marks on substrates that is inexpensive to manufacture and a method for reading the marks that can be easily integrated into a semiconductor manufacturing operation.